Adding test case.
sql_yacc.yy:
  Adding TEXT_STRING_filesystem, which
  converts from character_set_client to
  character_set_conversion.
  Replacing TEXT_STRING_sys to TEXT_STRING_filesystem
  in LOAD DATA and SELECT INTO OUTFILE contexts.
sql_class.h, sql_class.cc:
  Adding character_set_filesystem variable,
  and charset_is_character_set_filesystem
  flag (to avoid conversion when it's not necessary).
set_var.h, set_var.cc:
  Adding sys_var_character_set_filesystem
mysqld.cc:
  Adding --character-set-filesystem startup option.